1. Field of the Invention
The present invention relates generally to audio amplifiers, and more particularly, to eliminating pop-up noise in a class D audio amplifier from application of a power voltage.
2. Description of the Related Art
A class D amplifier modulates an audio signal to generate a pulse signal using pulse width modulation (PWM). The pulse signal is further processed by a low-pass filter, and a speaker generates sounds from the filtered pulse signal. The class D amplifier has higher efficiency than a class A amplifier, a class B amplifier, or a class AB amplifier, because the class D amplifier operates transistors only in a switching mode.
Thus, the class D amplifier has a high efficiency that is typically more than 80% at maximum output. In addition, the class D amplifier implemented with a silicon chip has a small size with low cost. Accordingly, the class D amplifier is widely used in low power applications such as mobile terminals, etc.
FIG. 1 is a block diagram of a conventional class D amplifier as disclosed in Korean Patent Laid-Open Publication No. 2003-74234. A signal source SIG generates an analog audio signal VIN that is centered on a ground voltage. The signal source SIG is connected to an input terminal TI of the class D amplifier 900 through an input condenser (not shown) for removing direct current (DC) components of the audio signal.
The class D amplifier 900 is a kind of a PWM (pulse width modulation) amplifier, and includes an input stage 901, a modulation circuit 902, a driving circuit 903, and n-type power metal-oxide semiconductor (MOS) transistors 904 and 905. The input stage 901 adjusts a reference point of the audio signal VIN to be suitable for input characteristics of the modulation circuit 902 operating at a power supply voltage VDD (for example, 10 V).
The modulation circuit 902 modulates an audio signal outputted from the input stage 901 into a pulse signal. The modulation circuit 902 converts a feature of the audio signal into a pulse width of a carrier signal to generate a pulse signal according to PWM (pulse width modulation). The driving circuit 903 complementarily controls the power MOS transistors 904 and 905 based on the pulse signal from the modulation circuit 902.
The power MOS transistor 904 is coupled between a positive power supply voltage VPP+ (for example, +50 V) and an output terminal TO. The power MOS transistor 905 is coupled between a negative power supply voltage VPP− (for example, −50 V) and the output terminal TO. The power MOS transistor 904 operates as a switch for charging the output terminal TO to a logic high state (i.e., VPP+) when turned on, and the power MOS transistor 905 operates as a switch for discharging the output terminal TO to a logic low state (i.e., VPP−) when turned on.
The output terminal TO is connected to an input terminal of a speaker SPK via a low-pass filter having an inductor L and a condenser C. A carrier frequency component of the power-amplified pulse signal at the output terminal TO is removed by such a low-pass filter. The filtered power-amplified pulse signal is an analog audio signal that is used by the speaker SPK to generate sound.
However, the conventional class D amplifier 900 of FIG. 1 removes pop-up noise generated during a volume control, etc., only during a normal operation state after a power supply voltage is applied. Another conventional class D amplifier 900 may remove pop-up noise generated during application of the power supply voltage using additional elements such as a relay inserted between the speaker SPK and the low-pass filter. In that case, the relay remains opened until internal operations of the class D amplifier are stabilized. However, including such a relay increases the size and cost of an audio device having the class D amplifier.
Japanese Patent Laid-Open Publication No. 2003-124749 discloses providing the pulse width modulated signal to a driving stage through an AND gate that is controlled by a mute signal for removing pop-up noise. However, although the pop-up noise may be removed effectively after the power supply voltage applied on the AND gate has stabilized, the pop-up noise may not be removed effectively as the power supply voltage begins to be provided.
In addition, according to Japanese Patent Laid-Open Publication No. 2003-204590, a mute signal may be generated after the applied power supply voltage has been stabilized for a short time. However, the mute signal may not be generated when the power supply voltage is slowly provided for a relatively long time, such that the pop-up noise is still generated.
Therefore, a low-cost and small class D amplifier with eliminated pop-up noise is desired.